Current engine technologies employ throttling mechanisms to control the amount of air inducted into an engine. These throttling mechanisms have inherent losses associated with them. Significant improvements in engine fuel efficiency can be obtained by reducing these engine throttling losses. Lean-burn engine designs, for example, take advantage of reduced throttling; however, nitrogen oxide(s) (NOx) emission control is a major issue associated with lean-burn engine technology. Lean-burn engines may also exhibit relatively high hydro-carbon (HC) emissions due to undesirable formation of unburnable lean air-fuel mixtures within the engine's combustion chamber.
Exhaust gas recirculation (EGR) is commonly used in current engines. Engines which implement EGR generally redirect exhaust gas through the engine's intake manifold to mix with an incoming fresh air charge. EGR provides some reduction in throttling losses and significant reductive impact on engine NOx emissions for part-load engine operation. Furthermore, EGR can be used with stoichiometric air-fuel mixtures to allow the use of conventional three-way catalysts for effective exhaust emission control.
The inventors of the present invention have found certain disadvantages with these prior art EGR systems. For example, EGR and the air-fuel mixture comprise a homogenous mixture. Excessive amounts of EGR result in poor combustion, poor vehicle driveability, and low fuel efficiency. Therefore, the fuel efficiency benefit is limited by the amount of EGR that the engine can tolerate. In addition, prior art EGR systems do not effectively reduce HC emissions because the crevice region (region between the piston and cylinder) is not effectively isolated from the air-fuel mixture. Further many current engines with prior art EGR systems have intake ports designed to enhance mixture motion to improve EGR tolerance. But these designs result in a compromise at wide open throttle conditions where nearly 100% air-fuel mixture (substantially no EGR) is required to enter the combustion chamber in a relatively unrestricted manner. In addition, some lean-burn engines operate in a stratified mode. Stratification is typically enhanced by uniquely shaped intake ports or secondary intake port throttling to cause a desired type of mixture motion. At wide open throttle, these uniquely shaped intake ports or secondary port throttles may undesirably restrict the air flow, resulting in reduced high speed volumetric efficiency. That is, at wide open throttle, mixture motion is not necessarily desired. Thus, intake ports, which induce less mixture motion, are desirable. This may be accomplished in the present invention because, at low engine speeds or loads, part of the mixture motion in the chamber is induced through the exhaust port. Thus a less restrictive intake port design may be utilized.